Remove index accesses from Azure provider

[csssuf]

Bug

coreos-metadata Version

1.0.0+

Environment

Azure

Expected Behavior

The Azure provider should not use index accesses, as they can cause panics.

Actual Behavior

The Azure provider uses index accesses here and here.

[sdemos]

More generally, I think it would be useful to audit the entire codebase for "things that can panic" and remove them wherever possible. I think we pretty much always want explicit error handling.